Inductively coupled tap



April 7, 1970 K. A. smoNs INDUCTIVELY COUPLED TAP Filed Jan; 21, 1969 3 Sheets-Sheet 1 m m ML ER M 0 SUCH .45 4/: 34 V/A COUPLERS SUCH AS .5

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United States Patent 3,505,615 INDUC'IIVELY COUPLED TAP Keneth A. Simons, 3011 Sycamore Road, Bryn Athyn, Pa. 19009 Continuation-impart of application Ser. No. $74,702, Aug. 24, 1966. This application Jan. 21, 1969, Ser. No. 814,482

Int. Cl. H011) 5/14 US. Cl. 333- 17 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND This application is a continuation-in-part of my application Ser. No. 574,702 filed Aug. 24, 1966, and entitled Inductively Coupled Tap, now abandoned. This invention relates to transmission line systems and, more particularly, to coupling arrangements between a'main transmission line or feeder line to one or more other transmission lines such as drop lines so arranged that such other transmission lines or drop lines may be added to the main or feeder line whenever desired to receive signals from the latter line.

In present transmission systems used for community antenna television systems, known as -CATV systems, so-called pressure taps have been used to connect the feeder line, usually in the form of a coaxial cable, to one or more drop lines, which are also coaxial cables and are expected to receive programs of TV and the accompanying signals. In such systems, a physical and electrical interconnection is made between the feeder line and each drop line, usually by means of a pressure tap which is individual to each drop line. The pressure tap involves a direct mechanical pressure contact to the center conductor of the feeder coaxial cable. The latter pressure contact is intended to establish a good electrical connection to the center conductor of the feeder. The pressure contact is in turn connected to the center conductor of the drop line coaxial cable usually through a series impedance elements or a transformer.

The installation of a pressure tap is usually made by a tool which is used to cut a small hole in the feeder cable shield and, thereafter, a pin is inserted under pressure into the center conductor of the cable to make electrical contact therewith. Experience with the pressure tap indicates that its performance has basic limitations.

' An object of the present invention is to provide an improved arrangement for coupling a feeder line to one or more drop lines without any physical or conductive connection to the center conductor of the feeder line.

A further objective of this invention is to provide an inductive coupler with substantially constant feeder-to-tap line loss and low insertion loss over a broad range of frequencies.

Another object of the invention is to provide a coupling arrangement such that signal energy may be fed from a feeder line to one or more drop lines, so arranged that the drop lines may be added to or disconnected from the feeder line from time to time as desired, without any physical connection to the center conductor of 3,505,615 Patented Apr. 7, 1970 the feeder line and without interfering with normal and continuous transmission over the feeder line.

Stated differently, an object of the invention is to provide mechanism for nonconductive connections between any or all of numerous drop lines to a feeder line or for disconnecting any or all of the drop lines, without appreciably affecting the quality of television or like service over the feeder line at any time, so that the feeder line will serve as a continuous reservoir of signal energy to be tapped by the drop lines as may be desired even while other drop lines are being added to or removed from the feeder line.

Another object is to provide a coupler for adding each of a plurality of TV drop lines to a feeder TV line so that there will be no appreciable return or feedback of disturbing signals to the feeder line from any of the plurality of drop lines to be added to such feeder line, both during the installation procedures and after the coupling has been completed, the arrangement being designed to make nonconductive connections between such feeder line and such drop lines and avoid any breakage or interruption of the feeder line or of the circuit between the feeder line and any and all of drop lines that were previously connected to the main TV line. In other words, the system is designed to maintain continuous transmission over such other lines during the intervals when the installation is to be made. And the object of the invention is to accomplish such interconnections quickly and simply and at very low cost.

This invention will be better understood from the following description when considered in connection with the accompanying drawings.

FIG. 1 illustrates schematically an arrangement for carrying out the principles of this invention.

FIG. 2 illustrates another schematic arrangement for the same general purposes.

FIG. 3 shows a sectional view of a general arrangement of one form of coil structure for coupling to a coaxial conductor transmission system of the type shown in FIGS. 1 and 2.

FIG. 4 is a graph illustrating the degree of coupling achieved by a configuration of the invention as illustrated in FIG. 3 over a range of frequencies from 0 to 300 mHz.

FIG. 5 is a graph of the degree of coupling achieved by a prior art capacitative type pressure tap, over a range of frequencies from 0 to 300 mHz.

FIG. 6 is a graph of the degree of coupling achieved by a prior art directional coupler physically coupled to the transmission lines, over a range of frequencies from 0 to 300 mHz.

FIG. 1 illustrates a main transmission or feeder line in the form of a coaxial cable of a familiar type which consists of a central conductor 1 and an outer shield 2, both of which are metallic and may be spaced from each other by a dielectric which may be, for example, in the form of any foam insulation. This coaxial cable may be used for carrying signals, usually a broad band of signals, such as one or more programs of TV signals together with the accompanying signals which may correspond to audible signals, etc. The coaxial cable system 1-2 may be used, for example, in a CATV system for feeding the usual TV programs and accompanying audio signals carried by the main or feeder line 1-2 to one or more drop lines, one of which is shown for illustration in the form of a similar coaxial cable consisting of a central conductor 3 and an outer shield 4 which may also be insulated from each other by a suitable dielectric. The coaxial cable 3-4 (together with other similar cables which need not be shown in the drawing) are designed to receive the signals carried by the main or feeder line 1-2 for supplying those signals to suitable receiving equipments located in the homes or offices of subscribers where such signals may be reproduced. The coupling network, shown schematically within the dotted line boundary 5, includes a coil 6 mounted on a core 7 of magnetic material and, preferably, a resistor 9 serving as a matching impedance.

The core 7 upon which the coil 6 is wound is inductively coupled to the central conductor 1 of the main or feeder line 1-2. There is no physical or conductive connection whatever between any part of the electromagnetic structure 6-7 and the central conductor 1. One terminal of coil 6 may be connected to the sheath or outer conductor 2 of the main or feeder line. The other terminal of coil 6 is connected to the central conductor 3 of the subscribers line 3-4 which is connected to receiving equipment such as a TV set (not shown). A resistor 9 is connected across coil 6 which is connected between the inner conductor 3 and the outer conductor 4 of the drop line 3-4. This resistor constitutes a terminating impedance providing back-match for the feed line 3-4.

FIGURE 2 is a simpler schematic of substantially the same general arrangement as that shown in FIGURE 1. Similar reference characters have been used to represent similar parts. The main or feeder line 1-2 is coupled to the drop line 3-4 through an electromagnetic structure consisting of coil 6 wound on core 7. The coil 6 and resistor 9 are both connected between the center conductor 3 and sheath 4 of the drop line 3-4, as shown. Obviously, the electromagnetic structure 6-7 may be fixed at any desired or predetermined angular position with respect to the center conductor 1 of the feeder line 1-2.

FIGURE 3 illustrates, by way of example, a sectional view of one form of the electromagnetic structure 6-7 shown generally in FIGURES 1 and 2. Sheath 2 has been cut away so as to accommodate the structure 6-7. The structure 6-7 is embedded in suitable insulation 11 as shown. There is a slot s in insulation 11 to enable the structure 6-7 to be moved over the central conductor 1. After insertion of the core, the slot s may be plugged with suitable dielectric material. The positioning and spacing of the core on the central conductor is therefore predetermined, and the core is protected against the weather. This construction would insure against any conductive connection between any portion of the coil 6 with any portion of the central conductor 1. Hence, any vibration or movement of the overall coupling structure would not affect the electrical coupling relation between the electromagnetic structure 6-7 with respect to the main transmission line 1-2.

In operation, TV or other signals flowing over the main or feeder line l-2 from left to right, for example, will be fed to the drop cable 3-4 so that the signals may be faithfully reproduced by receiving equipment connected at the distant end of the auxiliary cable 3-4. At the same time, the level of the signals passing through the point of the coupling mechanism to other points further along line 1-2 will be but minutely affected by the tapped connection to drop cable 3-4. This is due to the fact that the drop cable receives only a minor portion of the signal energy flowing along line 1-2. Hence, additional taps, which may be similar to the one shown and described as component 5 in FIGURE 1, may likewise be applied at other or more distant points along the main or feeder line 1-2, without amplification, for supplying other similar drop cable systems, so that subscribers sets connected thereto may likewise be arranged to reproduce the same signals faithfully.

As transmission takes place from left to right along the feeder line 1-2, the flow of current over the central conductor 1 will cause current to be induced in coil 6 which, at any instant, will generate a voltage corresponding to the signals traversing the feeder line 1-2. The voltage developed by the current induced in coil 6, will, with predetermined adjustments of the components of the coupling equipment 5, act to apply to the drop cable system 3-4 signals which correspond to those traversing feeder line 1-2. The subseribers television set connected at the distant end of line 3-4 will receive, and will be able to reproduce,

those signals with good fidelity. The voltage generated by coil 6 obviously may be adjusted by any well-known expedients of the art, such as by rotating the coil 6 with respect to the center conductor, or by moving coil 6 nearer to or farther from the center conductor, or by a variable slug within or adjacent to the core 7.

In a properly designed system, the transmission level at the receiver connected to line 3-4 may be in the range of 10 to 50 db lower than the level of signals that may traverse the feeder line 1-2. Such a comparative signal level will be suitable for transmission over the drop line of a plurality of TV programs occupying a broad band of the frequency spectrum as, for example, the band extending from the DC to the UHF range, including that part of the region extending from 54 mI-Iz. to 216 mHz.

In a coupler constructed according to the principles of this invention, the coupling was adjusted for a feeder-totap line loss of about 28 db. The loss inserted into the feeder line was measured at about 0.08 db and the return loss on the feeder line was measured at about 38 db. Typical values for a pressure tap of the prior art would be about 0.19 db insertion loss and about 30 db return loss, respectively, for a transformer tap with about 28 db feeder-to-tap loss.

Resistor 9 is connected between the central conductor 3 of the drop line 3-4 and ground as shown. This resistor 9 serves as a back-match or terminating impedance for line 3-4. This impedance is designed to match the normal characteristic impedance of line 3-4, for example 75 ohms. This back-match, or the impedance of resistor 9, will act to absorb reflections returned by irregularities in line 3-4 or by the subscribers set.

This invention departs greatly from any prior art atrangement which employed a multi-coil transformer or an auto-transformer. In these prior art arrangements, the transformer inherently caused a high impedance to be bridged across the feeder line. Specifically, in those devices employing transformers wound on ferrite cores, the loss characteristics of the cores resulted in an inherent shunt resistance which set a lower limit on the transmission loss introduced or introducible into the feeder line. This prior art is in great contrast to the device of this invention. In the present invention there is no lower limit to the transmission loss introduced into the feeder line. As the inductive coupling between the device 6-7 and the feeder cable 1-2 is decreased, the transmission between the feeder cable 1-2 and the drop line 3-4 is reduced without limit and, at the same time, the loss introduced into the feeder line is also reduced without limit. Thus, the device of this invention provides a tap having superior qualities particularly suitable for high feeder-to-tap losses. Furthermore, since the loss introduced into the feeder by each tap is reduced, this invention permits the use of longer feeders before the energy in the feeder is attenuated below a usable level.

Another advantage of the arrangement of this invention is the substantially flat frequency response or characteristic of the coupler between the feeder and drop lines. This frequency response can be made flat over a range extending from one or two mHz. to several hundred mI-lz. depending on the construction of the device 6-7, that is, its number of turns, its magnetic structure, etc. This response is flat regardless of the amount of magnetic coupling involved, so that efficient taps with high feeder-totap loss and flat frequency response area made practicable.

FIG. 4 is intended to illustrate that the output of the coupler of the invention is substantially flat" over a broad frequency range, and that this effect is achieved regardless of the distance of the coil from the center con ductor. FIG. 4 was obtained by measuring the output of a coupler of the invention, coupled to a test circuit. The coil utilized in the tests consisted of five turns of number 28 AWG insulated wire wound around the center of a U-shaped ferrite core inch in cross section. The ferrite core was dimensioned to fit within a recess cut into the side of a 0.412" diameter coaxial cable, as illustrated in FIG. 3. The arms of the U-s'haped core extended on either side of the center conductor 1 of coaxial cable 2 but not in contact therewith. The output of the coupler was then measured with the coil and core assembly placed at varying distances, indicated by the dimension a in FIG.

3, from the center conductor 1. Traces a, b, c and d on FIG. 4 represent the output of a test circuit of the coupler withjt'he coil 6 at four different positions varying in distance from center conductor 1. Traces x, y, and z of FIG. 4 represent the output of the test circuit with fixed attenuatops having the indicated attenuation, inserted in place of the coupler of the invention. The flat response characteristic of the coupler is evident from a comparison of traces a, b, c, and d to traces x, y, z of FIG. 4. Such a comparison reveals that a coupler constructed according to, the teachings of' the inve'intion provides a substantially constant output voltage at least over the range of frequencies utilized in CATV transmission, virtually irrespective of the positioning of the coil and the corresponding degree of attenuation. Such response is achieved in part by; keeping the capacity coupling between the coil and the center conductor to a minimum.

FIG. 5 is similar to FIG. 4 in which curve e represents the performance under test conditions similar to those under which FIGURE 4 was produced, of a prior art capacitive pressure tap of a type widely used in the CATV industry before the transformer tap was developed. It is evident by inspection of FIGURE 5 that the attenuation across the prior art tap varies directly with frequency. Such variation creates problems which are well known to those experienced in the art. The fiat frequency response of the coupler of the invention as? illustrated in FIG. 4 substantially eliminates such problems and the need for compensating devices which mustbe employed with a coupler having the characteristics exhibited in FIG. 5.

FIG. 6. depicts the performance of a prior art directional coupler of the coupled transmission line type, widely used at present under test conditions similar to those which produced FIGS. 4 and 5. As can be seen from curve g of FIGURE 6, this prior art coupler suffers from inescapable frequency response variation which limin its useful frequency to a frequency range of about 5 to 1; performance far below that of the coupler of the invention.

Inspection of FIGS. 4, 5, and 6 illustrates the uniformity .of response of the coupler of the invention over a broad range of frequencies. When the other advantages of the coupler of the invention, such as installation without cutting the main transmission cable or-substantially interrupting the flow of signals thereon, are added to the above advantages, it is evident that the invention has made a substantial and basic contribution to the art.

The reflection introduced into the feeder cable by the addition'of a tap is conveniently. measured in terms of return loss (which is the ratio ofthe reflected wave due to the tap; to the forward wave ialong the feeder cable expresscdfin decibels). In the arrangements of the prior art, the'rturn loss was limited .;to an undesirably low number by the shunt characteristics of the transformers used. In the present invention thereturn loss in increased without fignit as the tap couplingiis reduced. Thus, taps having high feeder-to-tap losses and having superior reflection characteristics are made practicable.

The core 7 of the electromagnetic structure may be made, for example, of ferrite material, but any other suitable core material, such as powdered iron, may be appropriate for this arrangement.

It is apparent that in the arrangement of this invention, compensating components such as condensers or other reactive or resistive components may be used in the usual way as would be apparent to those skilled inthe art.

While this invention has been shown and described with reference to a CATV system and to certain frequency ranges, the features of this invention are equally applicable to transmission systems for other types of signals and for other frequency ranges.

While this invention has been shown and described in certain particular embodiments merely for illustration, other embodiments will be apparent to those skilled in the artrl'lence, the invention is to be treated as limited only by the scope of the appended claims.

What is claimed is:

1. A cloupler for coupling two lines to each other, one of which; is a coaxial cable formed by a longitudinal conductor positioned at the center of a metallic she'ath, comprising coil mounted on a core and arranged as an inductive element, but having no conductive connection to the ce nter conductor, for receiving signals corresponding to currents flowing over the coaxial cable, the sheath of said cable being cut away so as to house said-coil and said core, the other line being connected to the'terminals of said 9a for receiving a portion of the signals traversing the coaxial cable.

2. A boupler for coupling a line to a coaxial cable so as to extract signal energy from said cable for transmission .over said line, comprising a core of magnetic material" adjacent to the longitudinal conductor at the center of said cable, a coil wound upon saidfcore, the terminal; of said coil being connected to said line, said coil having no conductive connection to the longitudinal conductgr but constituting the sole means for coupling said cable to said line.

3. A coupler in accordance with claim 2 in which the sheath of the coaxial cable is cut away so that the core and the'coil may be housed within said sheath.

4. A coupler in accordance with claim 3 in which the coil and the core are embedded in insulation and are packaged together so as to be inserted within the sheath of said cable.

5. A coupler for coupling a subscriber's line to a coaxial cable carrying currents corresponding to one or more television programs to be transmitted .to distant points, comprising a core which is adjacent to the central conductor of the cable, a coil wound upon said core but having no conductive connection to the central conductor, and aresistor, said coil and said resistor being separately connected across the subscribers line, said resistor constituting the terminating impedance for waves flowing over said line toward the cable.

6. Agcoupler according to claim 5 in which the core and the. coil are embedded in insulation and constitute {he sole coupling elements between the cable and the 7. A coupler according to claim 5 in which the core, the coil and the resistor are enclosed within a common housing which is positioned adjacent to the coaxial cable and the end of the subscriber's line at their point of interconnection.

8. A coaxial cable feeder system comprising a coaxial cable for feeding TV programs or other signals to distant points, fa U-shaped core, a coil wound upon said core, the sheath of said coaxial cable being cut away for receiving said cell and said core, a line connected to said coil for receiving therefrom a portion of the signals traversing the coaxial cable, said coil and said core constituting the sole coupling means between said cable and said line.

9. A coaxial cable feeder system in accordance with claim 8 which also includes a terminating impedance bridged across said line at a point adjacent to the coaxial cable, said terminating impedance constituting a backmatch absorbing reflected wave returned from the line.

10. A coaxial cable feeder system in accordance with claim 8 in which said coil and said core are embedded in insulation and are inserted within the sheath of said cable.

11. A coaxial cable feeder system in accordance with claim 10 in which the core is composed of a ferrite material.

12. An inductive coupling between a drop line connected to a receiver and a main coaxial signal carrying cable having a center conductor, and an outer conducting sheath insulated from said center conductor, the coupler being installable without breaking the circuit of the main cable and without substantially interfering with the flow of signals therethrough, comprising; a coil wound upon a ferrite core and positioned within but apart from said sheath and adjacent to but apart from the center conductor of said cable, so as to be inductively coupled to said center conductor, said drop line connected across said coil, and insulated means to retain said coil in said position.

13. The coupling of claim 12 having a portion of said sheath removed from said cable to provide access for said coil.

14. The coupling of claim 13 further comprised of an impedance connected across said coil, said impedance matching that of said drop line.

15. An inductive coupler for coupling two lines to each other, each of which is a coaxial cable comprised of:

a coil wound upon a U-shaped ferrite core, the terminals of which are connected to the second coaxial cable, said coil being physically inserted into said 2 first coaxial cable, connected to the sheath but not to the center conductor thereof,

8 the reactance of said coil over a seiected broad fre quency band being substantially higher than the characteristic impedance of the second coaxial cable. 16. The coupler of ciaim 15 wherein said coil is positioned with the legs of said U-shaped ferrite core extending over opposite sides of the center conductor of the coaxial cable.

17. The coupier of claim 16 wherein said coil is positioned in a plane substantially normal to the axis of the coaxial cable.

References Cited UNITED STATES PATENTS 2,611,895 9/1952 Lacey 333-8 X 2,762,016 9/1956 Bradburd et ai. 333-24 X FOREIGN TATENT-S 1,146,559 4/1963 Germany.

HERMAN K. SAALBACH, Primary Examiner P. L. GENSLER, Assistant Examiner 5 Us 01. X.R. L 325-308; 333-44,- 343-858 

